Open Graded Asphalt Surfacing

OGA is porous. Therefore OGA must be placed on an impervious layer or layers to stop water penetrating underlying pavement layers. It is important to note that the PDM (QDMR 2009) requires:

• the use of a PMB seal at the bottom of all surfacing layers; and

• a 50 mm thick DG14 layer immediately below the PMB seal under an OGA surfacing.

Reference should also be made to Section 4.11 which deals with the use of polymer modified and multigrade binders.

4.4.9.1 Functions

The main functions of OGA surfacings are as follows:

• Provide a surfacing with an appropriate level of skid resistance and surface texture.

• Provide a surfacing that will minimise the likelihood of aquaplaning.

• Provide a surfacing that minimises water spray thrown up by vehicles.

• Provide a surfacing that is quieter than DGA, SMA, concrete or bituminous seal surfacings.

While it is not the main reason for selecting OGA, a possible ancillary benefit is that it can also reduce water spray, and so improve visibility, in wet weather.

4.4.9.2 Appropriate Uses

OGA can be used as a surfacing only where the underlying pavement is sound. In addition, the following must be noted:

• OGA surfacings have also been used successfully on bleeding surfaces where excess bitumen was able to fill the voids without flushing the surface. However this may compromise its functions (see Section 4.4.9.1) and in this application it may only be a short term fix.

• Provision must be made to allow free flow of water through the layer (e.g. free edge is proud of kerb and channel, transverse joints with DGA drain).

• Surfaces on which an OGA surfacing is placed must be reasonably smooth and free draining. Surface irregularities or depressions must be corrected prior to the construction of an OGA surfacing. This may include shape correction to milled surfaces.

4.4.9.3 Inappropriate Uses

Inappropriate uses of OGA include the following:

• Where the underlying pavement is unsound or poorly drained.

• Where the underlying asphalt contains water.

• As anything other than a surfacing.

• In areas subject to high shear forces (e.g. tight curves, heavy braking areas).

• Where it is not placed on a PMB seal that is itself placed on a DGA layer. Refer to the PDM (QDMR 2009) for further details.

• As a treatment to improve structural capacity.

• As a waterproof surfacing.

• To correct the shape of the pavement.

• Where there is reflective cracking and it is not combined with appropriate treatments such as a SAMI or a minimum of 175 mm (total thickness) of DGA (see also Section 4.9.6.3).

• Over OGA surfacing.

• Over SMA unless it is proven to be in sound condition and its moisture content is no greater than 0.5%. The overlaying of SMA is discussed further below.

• Over unsound asphalt. This should be removed or repaired as required before the overlay is constructed.

• Where cracks in underlying cement treated layers or concrete pavements are not treated (e.g. see Sections 4.4.4 and 4.8).

• Over surfaces that have depressions, rutting or other shape deficiencies corrected. Depressions, rutting, etc will pond water within the OGA layer (i.e. inhibit drainage through and out of the layer) and can lead to pavement failures. Such shape deficiencies need to be corrected (e.g. with a DGA corrector course) prior to overlay).

• Over milled surfaces without a DGA corrector course. Grooves collect water and keep it within the OGA layer, even with a seal underneath the OGA surfacing. Where the milling is very fine (e.g. a fine milling drum with a teeth spacing not exceeding 8mm is used) this may be less of an issue.

• For hardstand areas (e.g. heavy vehicle parking areas).

Some earlier SMA surfacings were permeable and held water. Stripping of the existing SMA may therefore be an issue for older SMAs, particularly if they were constructed to the department’s superseded MRS11.33 (12/99) specification for SMA or specifications older than MRS11.33 (12/99). Therefore it is recommended that the sealing over or the overlaying of an existing SMA is only considered where:

• it is proven to be in sound condition by an appropriate amount of coring and testing of it;

• its moisture content is no greater than 0.5%.

4.4.9.4 Materials

Relevant TMR specifications and technical standards include the following:

• MRTS18 Polymer Modified Binder (TMR 2009d).

• MRS30 and MRTS30 Dense Graded and Open Graded Asphalt Pavements (TMR 2010h).

• MRS31 and MRTS31 Heavy Duty Asphalt (TMR 2011a).

4.4.9.5 Design Considerations

OGA surfacings make a minor contribution to structural strength. However they are mainly used due to their other properties (e.g. porous nature).

Where the speed limit is greater than 80 km/h and asphalt surfacing is required it is recommended that an OGA surfacing be used

Where the water flow paths are long some of the desirable properties may be lost. For example on a three lane carriageway with one way crossfall falling to the left, the leftmost lane may become saturated with water in wet weather meaning there may be no or minimal spray reduction in the outer lane. To mitigate this OG14 is recommended where there is a carriageway with three or more lanes with one-way crossfall.

4.4.9.6 Construction Considerations

The following additional factors need to be considered:

• The construction requirements specified in MRTS30 and MRTS31 (TMR 2010h, 2011a).

• It is desirable to limit the time of haulage, to reduce the chances of binder draining to the bottom of the truck bed.

• OGA mixes not only are difficult to place and work by hand, but cool rapidly, so that the time taken to spread it by hand may result in a lack of bond between the particles of aggregate.

• A thin layer compresses very little under the roller and, as it cools quickly, it must be rolled as soon as possible.

• Evidence of binder drain down also includes blotches in the finished OGA surface.

• Placement of OGA prior to the mix cooling below the minimum rolling temperature, it is likely that the OGA surfacing will ravel prematurely and require more frequent resurfacing.

4.4.9.7 Expected Performance or Comments

Based on the field experience with OGA surfacings, the following are some of the major points to note with respect to their performance:

• OGA surfacings should be laid in thin layers and with a consistent thickness. Any shape correction required should be done before a PMB seal and the OGA is laid (e.g. with a DGA corrector course).

• It is porous which means any liquid spilt on it will enter the matrix. Depending on the fluid this can be detrimental to the OGA. For instance a petroleum spill can cause rapid deterioration of the materials and as a consequence the surfacing. In such a case the only solution is removal and replacement.

Note however that patching is problematic (see below).

• It is difficult to undertake isolated repairs (e.g. patch) OGA surfacings. For example a DGA patch can not be used to repair an isolated failure in an OGA surfacing. Further it is difficult to get small quantities of OGA, especially if the repairer attempts to match the original OGA mix design.

• Line marking paint may change the properties of the OGA surfacing over the area of line marking (e.g.

reduced surface texture over line and/or may block pores).

• If the OGA surfacing is not carried across the full width of the carriageway (e.g. shoulders not included) a ‘drop-off’ will result. Consideration needs to be given about whether this is an appropriate approach and, where appropriate, what maximum height of drop-off is acceptable for the project, where drop-offs are located and whether the portion not receiving an overlay requires any other sort of treatment (e.g. reseal). This needs to include consideration of whether the use the road by cyclists is significant and how the drop-off will affect them.

• Tapers, often in conjunction with cold planning, are usually required to match into road furniture that is not raised (e.g. kerb and channels) or to match existing surface heights of the adjoining road(s) (e.g.

at each end of the works). Drainage of the OGA at joints with adjoining pavements needs to be addressed.

• Placement by hand and hand work often creates a different surface texture when compared to OGA laid by a paver.

• The expected service life of OGA is shorter than that of DGA surfacings.

• Early preventative maintenance is essential.

• The pavement remains wet longer.

4.4.9.8 Further reading

Relevant publications include the following:

• The PSM (TMR 2011b).

• The PDM (QDMR 2009).

• AGTPT Part 3: Pavement Surfacings (Austroads 2009a).

• AGTPT Part 9: Pavement Work Practices (Austroads 2008c).

• AGTPT Part 4B: Asphalt (Austroads 2007).

• Guide to the Selection and Use of Polymer Modified Binders and Multigrade Bitumens (Austroads 2006b).

• Pavement Work Tips (Austroads and AAPA various).

• Technical Note 28: Material Transfer Vehicles in Asphalt Paving towards Smoother and Longer